Fuel cells have been used as a power source in many applications. Some fuel cells (e.g. PEM-type or phosphoric-acid-type) use hydrogen supplied to the anode as fuel, and oxygen supplied to the cathode as oxidant. The hydrogen can be provided directly from liquefied or compressed hydrogen, or indirectly from reformed hydrogen-containing fuels such as methane, methanol, gasoline or the like. The oxygen is typically provided from air.
PEM fuel cells are preferred for vehicular applications (i.e. as replacement for internal combustion engine) owing to their compactness, moderate temperature operation, and high power density. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, protontransmissive, solid polymer electrolyte membrane (e.g. perfluoronated sulfonic acid) having an anode catalyst on one of its faces and a cathode catalyst on its opposite face. The MEA is sandwiched between a pair of electrically conductive current collectors that distribute the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode. See U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
For vehicular applications, it is desirable to use a liquid, hydrogen containing fuel such as methanol or gasoline as the source of hydrogen, because it is easy to store onboard, and there is an existing, nationwide infrastructure capable of supplying such fuels. However, hydrogen-containing fuels (liquid or gaseous) must be dissociated to release their hydrogen content. The dissociation reaction is accomplished within a chemical reactor known as a fuel processor. Fuel and water vaporizers located upstream of the fuel processor convert water and liquid fuel to steam and fuel vapor respectively for supply to the fuel processor. The fuel processor contains one or more catalytic reactors wherein vaporized fuel reacts with steam, and sometimes air, to yield a hydrogen-rich reformate gas that is supplied to the fuel cell. In the steam reformation of methanol, methanol vapor and steam are reacted. In the reformation of gasoline, steam, air and gasoline vapor are reacted in a fuel processor known as an autothermal reformer (ATR). The ATR contains two sections, i.e. (1) a first, partial oxidation (POX) section where the gasoline is partially reacted exothermically with air, and (2) a second steam reformer (SR) section where the effluent from the POX section is exothermically reacted with steam. The effluents from both the methanol and gasoline reformation processes contain primarily hydrogen and carbon dioxide along with some water and CO. A water-gas-shift (WGS) reactor and/or a preferential oxidation (PROX) reactor may be provided downstream of the reformer to reduce the CO content of the hydrogen stream. See U.S. Pat. No. 6,232,005 (issued May 15, 2001), U.S. Pat. No. 6,077,620 (issued Jun. 20, 2000), and U.S. Pat. No. 6,238,815 (issued May 29, 2001), each assigned to the General Motors Corporation.
For vehicular applications, the fuel processor must be capable of delivering hydrogen to the fuel cell over a wide range of rates depending on the power demands placed on the fuel cell. In this regard, the fuel cell has to be able to power the vehicle over a broad spectrum of operating conditions ranging from when the vehicle is standing at idle (i.e. a low power demand condition) to when the vehicle is moving at high speeds (a high power demand condition), as well as transitions (e.g. rapid acceleration) therebetween (a very high power demand condition). Heretofore, the ability of the fuel processor to quickly respond to very high power demand transitions between low and high power demand conditions (hereafter “power surges”) of the fuel cell has been hampered by the ability of the system to quickly produce enough vaporized fuel and/or steam for supply to the fuel processor. In this regard, it has not been considered practical to design a vehicular fuel cell system with vaporized fuel and steam reserves sized to accommodate power surges. Rather, the systems are typically sized to accommodate substantially steady state low and high power conditions, but not the power surges therebetween. Hence, transient response has suffered.